Heroes against gunslingers.

Ways i have handled guns in D&D and the ways they enhance or reflect Narrative and mood.
They don't have to conflict with the narrative, they can enhance it when used right. And no they aren't ALWAYS an excuse to be ultra powerful. 
Amd again, you can just say, "I don't want guns so i won't run them." 

1: Rare, special and powerful: This meant that every round was a carefully used resource. The party was getting paid for retrieving the weapon, but also a bonus for every round they brought back. A few times it came down to "Do we make sure we survive this encounter or do we risk it and save 1000GP by using sword and spell?" 
Led to some tense navagation of situations, and the players engaged way differently than normal. Seemed to love it except for one player, but I gave him his 'cake' when I could.

2: A novel mystery: Had the players fight and defeat a Yautja (Predator) and they managed to get his gear. Since it was a party of mostly spellcasters, i described them like magic items in their function but would keep saying they weren't magical. They then spent a few session studying them in their downtime until one of the players clocked what it was they fought. They then tried to use them until they ran out of power. 
They managed to use it to approach something in a very novel way and rescue a princess earlier than I intended by using the Cloaking device since it wasn't magical and didn't set off detect magic stuff. A new player to the table then joined and played the Princess as a fighter.

3: Reskin: This has been drought up a bunch of times already. It is simple aesthetics. The Ranger was actually a modern-day hunter who had been Isekaied into Faerun, and his rifle was just a bow that made more noise (his call.) We handwaved where he kept getting ammo, and he just played along with everyone else. It would only come up when he wanted a disadvantage, like it being VERY LOUD. This worked for all his other gear. 
When the party got back to 1990s earth, the Cleric and the Wizard were more powerful than every threat they encountered.
 Turns out a modern day bar brawler isn't eager to mess with someone who can cause 5 foot sections of air to become freezing cold.

4: Underwhelming Novelty: Yeah that incindiary grenade was neat, until it was mage hand swept into the river. Or the monk catches it and throws it back at you. But it inspired the wizard to show off his version. FIREBALL!
Neat assault riffle. Too bad its ammo is explosive and the Bard is doing heat metal on your spare magazines. 
Flamethrower? Well the Barbarian is on fire, but he still seems to be running at you with an axe, while he and the axe are on fire, and he wants a hug. 

There are ways to do it that don't need official support, can be tailored to the feeling you want. Realism is a rabbit hole that can only lead to madness of comparing muzzle velocities and kinetic energy potential at various ranges, so don't bother with it unless you want to inevitably end up with the "Russian SKSs are better than Chinese SKSs so mine should do more damage" blah blah blah pile of unfounded opinions.